Neutron capture cross sections of Kr

Stefan Fiebiger,Milan Krtička,Bayarbadrakh Baramsai,Mario Weigand,John Ullmann,Shea Mosby,John O’Donnell,Gencho Rusev,Aaron Couture,Clemens Wolf,René Reifarth,M La Cognata,C Spitaleri,R.G Pizzone,S Palmerini,M Lattuada

doi:10.1051/epjconf/201716501023

Abstract

Neutron capture and β− -decay are competing branches of the s-process nucleosynthesis path at 85 Kr [1], which makes it an important branching point. The knowledge of its neutron capture cross section is therefore essential to constrain stellar models of nucleosynthesis. Despite its importance for different fields, no direct measurement of the cross section of 85 Kr in the keV-regime has been performed. The currently reported uncertainties are still in the order of 50% [2, 3]. Neutron capture cross section measurements on a 4% enriched 85 Kr gas enclosed in a stainless steel cylinder were performed at Los Alamos National Laboratory (LANL) using the Detector for Advanced Neutron Capture Experiments (DANCE). 85 Kr is radioactive isotope with a half life of 10.8 years. As this was a low-enrichment sample, the main contaminants, the stable krypton isotopes 83 Kr and 86 Kr, were also investigated. The material was highly enriched and contained in pressurized stainless steel spheres.

Highlights

Most of the elements with proton numbers higher than iron are produced through neutron-induced processes
If the conditions in the star make the rates for neutron capture comparable to the rate of β-decay for a particular isotope, the s-process path branches at that isotope: a fraction of the isotope transforms via neutron capture, while the other fraction β-decays
The branching ratio, or relative likelihood, for the different reactions depends on the physical conditions in the interior of the star – temperature, neutron density and electron density

Summary

Introduction

Most of the elements with proton numbers higher than iron are produced through neutron-induced processes (neutron captures). A few isotopes on the proton-rich side of the valley of stability get significant contributions from other processes. If the conditions in the star make the rates for neutron capture comparable to the rate of β-decay for a particular isotope, the s-process path branches at that isotope: a fraction of the isotope transforms via neutron capture, while the other fraction β-decays. The branching ratio, or relative likelihood, for the different reactions depends on the physical conditions in the interior of the star – temperature, neutron density and electron density. At higher neutron densities with all other conditions equal, more nuclei of a given isotope capture a neutron before having the chance to β-decay. There is isomeric state with half life of 4.48 h which always decays before neutron capture – either through β-decay or through internal conversion to the ground state

Samples

Experiment Analysis